Generally, a plasma generating electrode is used in waste or drinkable water treatment, such as sterilization of microorganisms, removal of organic or inorganic contaminants, e.g. Volatile Organic Compounds (VOCs), or the like, or is used as a underwater sound generating source.
FIG. 1 is view showing a conventional plasma discharge apparatus used in a common liquid medium. The conventional plasma discharge apparatus includes: a main body 1 that is filled with liquid (a liquid medium); a flat ground electrode 2 which is provided at one side within the main body; a needle or rod type power electrode 3 which is disposed in the main body opposite the ground electrode 2; and a high voltage power supply device 4 which serves to supply electric power to the power electrode 3. The power electrode 3 is coated with an insulating material 5. A dotted circle in FIG. 1 is the region where corona discharge, sparks, or arc discharge occurs.
However, such a plasma discharge apparatus has problems of being difficult to be made larger, of reduced efficiency, and of being difficult to obtain a permanently-operable power supply device. In addition, the plasma discharge apparatus also has limitations of short life of an electrode and of lower adaptability that it can only be applied to the liquid medium (e.g. ultra pure water) having very low conductivity.
FIG. 2 is a view explaining the liquid medium plasma generating wattage when using the conventional electrode structure. The liquid medium plasma generating wattage of the plasma discharge apparatus having the conventional electrode structure will now be described with respect to FIG. 2.
A simple equation for obtaining the plasma generating wattage is as follows:Electric field strength E=V/d 
Here, V is voltage, and d is a length of conductive volume.V=I×R 
Here, I is conduction current, and R is resistance across electrodes.I=V/R R=d/A×S 
Here, A is a cross-sectional area of conductive volume, and A is electric conductivity of a liquid medium.Wattage W=V×I 
Assuming that the liquid medium is super pure water, the length (d) of the conductive volume is 1 cm, the cross-sectional area (A) of the conductive volume is 2×2=4 cm2, and the conductivity of the ultra pure water is 50×10−6 (S/cm), the conductive resistance (R=d/A×S) becomes 1/(50×10−6×4)=5000 (Ω). Here, if the electric field strength E for generating plasma discharge in the ultra pure water equals 5 kV/cm, required voltage (V=E×d) becomes 5 kV/cm×1 cm=5 kV. However, if electric conduction occurs through ultra pure water, conduction current (I) equals 5000 (V)/5000 (Ω)=1 (A), and the wattage (W) equals 5000 (V)×1 (A)=5 (kW).
Next, assuming that the liquid medium is sea water, the length (d) of the conductive volume is 1 cm, the cross-sectional area (A) of the conductive volume is 2×2=4 cm2, and the conductivity of the sea water is 53×10−3 (S/cm), the conductive resistance (R=d/A×S) becomes 1/(53×10−3×4)=4.7 (Ω). Here, if the electric field strength E for generating plasma discharge in the sea water equals 5 kV/cm, required voltage becomes 5 kV. However, if electric conduction occurs through sea water, conduction current (I=V/R) equals 5000 (V)/4.7 (Ω)=1064 (A), and the wattage (W=V×I) equals 5000 (V)×1064 (A)=5.3 (MW), which corresponds to total wattage consumed by a small city. However, such a power supply device does not exist, nor is impossible to realize even using a pulse. Thus, using such an electrode structure cannot generate plasma discharge through the sea water.